1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive comprising a VCM stall detector for velocity control of an-actuator arm.
2. Description of the Prior Art
Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
There are times when the servo control system does not have access to the embedded servo sectors yet it is still desirable to control the velocity of the actuator arm. For example, in disk drives wherein the head is parked on a landing-zone of the disk, it is desirable to control the velocity of the actuator arm to unlatch the head during spin-up. In disk drives employing ramp loading/unloading, it is desirable to control the velocity of the actuator arm so that the head is not damaged as it travels off the ramp onto the disk as well as off the disk onto the ramp. Another example is if the servo control system loses servo sector synchronization it is desirable to control the velocity of the actuator arm to facilitate re-synchronizing to the servo sectors.
Prior art techniques for controlling the velocity of the actuator arm when servo sector information is unavailable include using a voltage loop with the detected back EMF generated by the VCM as the feedback. The VCM is essentially an RLC circuit where R is resistance, L inductance, and C the inertia of the motor and load. The voltage contribution of C to the measured back EMF is proportional to the velocity of the VCM. Since the resistance R is in series with C, it is desirable to cancel R's contribution to the back EMF leaving only LC. Once the resistance R is canceled, at low frequencies Ldi/dt is small leaving the voltage contribution of C as the dominant factor in the measured back EMF.
Prior art techniques for performing VCM resistance compensation include calibrating and subtracting from the measured back EMF the voltage contribution of R (i.e., the IR voltage where I is the current in the VCM). The VCM resistance R is measured by applying a fixed current to the VCM in order to press the actuator arm against a fixed object (e.g., the crash-stop for stopping the head at the inner diameter (ID)). With the actuator arm pressed against the fixed object, the velocity is zero and Ldi/dt is zero, leaving the VCM resistance R as the only contribution to the measured back EMF.
If the IR voltage is completely canceled from the measured back EMF, it would result in an under-damped or unstable system. Thus, it is desirable to decrease the measured VCM resistance R by a small offset to leave a relatively small amount of IR voltage in the measured back EMF. It is also desirable to detect when the motor stalls to prevent overheating and overstressing the VCM. The VCM will stall, for example, at the end of an unload (or parking) operation when the head latches, at the beginning of a load (or unparking) operation if the head fails to unlatch, or if the drive fails to synchronize to the embedded servo sectors during a load operation and the actuator arm crashes into the ID crash-stop. Detecting a failure to unlatch during a load (unpark) operation also saves time during drive initialization. However, because a small amount of VCM resistance R is not canceled to achieve an over-damped system, when the VCM stalls the measured back EMF will actually rise giving a false indication that the actuator arm is still moving.
There is, therefore, a need to detect VCM stall conditions to prevent overheating and overstressing the VCM, while still allowing a small amount of effective VCM resistance R to achieve an over-damped velocity controlled system.